1. Field of the Invention
The invention relates to rolling cutter drill bits for drilling holes in subsurface formations, and particularly to the design and clearances of the internal bearing structures for such bits.
2. Description of Related Art
As is well known in the art, a rolling cutter drill bit typically comprises a bit body including a plurality of lugs, usually three, each of which includes a journal on which a rotating cutter is supported by suitable bearings. The cutters rotate relative to their respective journals, as the bit is rotated within an earth formation, to perform a cutting action on the formation. Each cutter is secured to its journal by means of a retention assembly, and typical forms of such assembly are shown in U.S. Pat. Nos. 4,838,365 and 5,080,183. A small amount of axial play between the cutter and journal is required to facilitate the appropriate rotating action of the cutter, and to prevent binding of the cutter as a result of differential thermal expansion. The retention assembly must therefore be designed to allow some minimum degree of relative axial displacement or play between the cutter and journal.
A rolling cutter bit normally includes a lubrication system to provide lubricant to the bearings between the cutter and the journal in the cutting assembly. These lubrication systems typically include a lubricant reservoir within the bit from which lubricant is supplied to the bearings, and means for pressure balancing the lubricant relative to the environment exterior to the bit. In order to maintain the lubricant within the bit, a seal assembly is provided to seal between the rolling cutter and the stationary journal. Various forms of seal assembly are described and shown in U.S. Pat. Nos. 3,137,508, 3,761,145, 2,590,759, 4,466,622, 4,516,641, 4,838,365 and 5,080,183.
The prior art has established that axial play of the rolling cutters in sealed and lubricated drilling bits causes significant lubricant volume transfers inside the cutter bearing, particularly near the seal. These volume changes lead to high pressure differentials across the seal which limit seal reliability and ultimately limit the useful life of the bit. In recognition of these problems, the prior art focused upon seal designs to tolerate these unwanted pressure fluctuations. For instance, in U.S. Pat. No. 3,137,508 it was recognised that pressure differentials of up to 50 psi can appear at the seal at the rate of 1800 fluctuations per minute. Thus, a seal was invented which leaked a small amount of lubricant outwardly in response to excess internal pressure. In U.S. Pat. No. 3,761,145 a rigid face seal design was disclosed which also was designed to leak lubricant to limit internal pressure inside the rolling cutter.
Another type of rigid face seal shown in U.S. Pat. No. 2,590,759 was designed to move axially to compensate for lubricant volume fluctuations rather than release lubricant. Somewhat similar volume compensating rigid face seal designs are shown for drill bits in U.S. Pat. Nos. 4,466,622 and 4,516,641. In particular, U.S. Pat. No. 4,516,641 discusses at length exactly how much axial displacement of the seal is required for a given amount of axial play in the rolling cutter. There are many other patents for drill bits which disclose seal designs which better tolerate the pressure fluctuations of the lubricant. One commonality throughout these inventions, however, is that the presence of these pressure fluctuations is detrimental to bit life.
As shown in the prior art, many factors combine to cause pressure fluctuations in face seal assemblies, however, the one factor that drives the rest is the permitted axial play between the rotating cutter and the journal which carries it. If axial play were to be zero for the life of the bit, there would be no volume changes to drive pressure fluctuations. However, as previously explained, the design of the bit must always provide some minimum degree of axial play.
A common problem of bits incorporating rigid face seals is inconsistency of performance. Our belief is that prior designs for rigid face seals in rock bits concentrated on the seal assembly design with less regard to the other factors. In particular, the critical design factor affecting seal life, i.e. the maximum permitted axial displacement of the cutter with respect to the journal on which it is mounted, has been allowed to vary considerably from one assembly to the next during manufacture.